1. Field of the Invention
The present invention relates to a sheet feeding apparatus, such as an automatic document feeder, provided mainly on a copying machine and an image reading apparatus.
2. Description of the Related Art
The document feeder attached to conventional document reading apparatuses drives a motor to lower a pickup roller which is waiting ready at a standby position distant from the uppermost sheet loaded on a document positioning plate, in synchronization with a sheet feeding start signal. The pickup roller comes in contact with the uppermost sheet on the document positioning plate and then is rotatively driven to feed the uppermost sheet. When the leading edge of the uppermost sheet reaches a sheet detection sensor disposed on the downstream side in the sheet conveyance direction, the document feeder drives a motor to raise the pickup roller to the standby position to be ready to feed the following sheet.
When the uppermost sheet is conveyed by a conveyance roller disposed on the downstream side and then the document feeder detects that the trailing edge of the sheet has passed the sheet detection sensor, the document feeder starts feeding the second sheet. More particularly, after being lowered, the pickup roller rotates to feed the second sheet. The document feeder repeats a similar operation to feed the third and subsequent sheets.
With respect to the copying machine, an automatic zooming function is known. This function compares the sheet size to be read by the document reading apparatus with the size of a recording sheet set in a printer unit, and enlarges or reduces the size of the read image to fit it into the size of the recording sheet for image formation. To perform this function, it is necessary to set the magnification ratio at the zooming processing unit before reading a document image. To set the magnification ratio, it is necessary that the sheet size is known before reading the document image.
To detect the sheet length, in a conventional method, the sheet length is calculated based on timing at which the sheet detection sensor disposed on the downstream side in the sheet conveyance direction detects the leading and trailing edges of the sheet, and the conveyance speed.
This method, however, determines how far the leading edge of the sheet has been fed toward the downstream side of the trailing edge when the sheet detection sensor detects the trailing edge of the sheet. Therefore, the conveyance path length required by the method is such that the leading edge of the sheet has not reached the reading position when the sheet detection sensor detects the trailing edge of the sheet. As a result, there has been a problem that the document feeder cannot be downsized.
Particularly in a case of a compact copying machine, it is also necessary to downsize the document feeder mounted on the document reading apparatus. However, since reducing the conveyance path length of the sheet conveyance apparatus makes it impossible to detect the sheet length before the leading edge of the sheet reaches the reading position, there arises a problem that the automatic zooming function cannot be implemented.
If the image reading apparatus starts image reading before the sheet size is determined, the magnification ratio is not determined and therefore it is necessary to once store the read image in memory. In this case, since the sheet size has not been determined, the document feeder constantly requires image memory which fits the maximum conveyable sheet size, resulting in cost increase.
Further, since the sheet size is not determined until the trailing edge passes the sheet detection sensor disposed on the downstream side of a sheet separation position, the timing of sheet size detection is delayed, preventing the increase in operating speed of the apparatus.
A method discussed in Japanese Patent Application Laid-Open No. 09-301573 provides a roller rotating in response to the movement of the uppermost sheet on a document tray of the document feeder, a disc coaxially rotating with the roller, and a photointerrupter which detects holes on the disc. While the uppermost sheet is moving, the roller is rotating and a signal is output from the photointerrupter. When the trailing edge of the sheet has passed the roller, the roller stops rotating and the signal is no longer output from the photointerrupter. When the signal is not output from photointerrupter, the document feeder detects the trailing edge of the sheet. Then, after a fixed time has elapsed since the document feeder detected the trailing edge of the sheet, it starts lowering the pickup roller. In this way, the trailing edge of the sheet can be detected at earlier timing than the conventional method for detecting the trailing edge by use of the sheet detection sensor disposed on the downstream side after sheet separation.
However, the method discussed in Japanese Patent Application Laid-Open No. 09-301573 cannot strictly determine whether or not the trailing edge of the sheet has passed the sheet detection sensor because the roller rotating in response to the movement of the sheet may rotate because of the inertia of the body of revolution even after the trailing edge has passed the sheet detection sensor. Further, depending on the pitch of the holes (light-blocking portions and non-light-blocking portions of the photointerrupter) on the rotating disc, the trailing edge of the sheet is detected after a certain amount of time has elapsed since the trailing edge of the sheet actually passes the sheet detection sensor. Therefore, even if the photointerrupter stops immediately after the trailing edge has passed the sheet detection sensor, the actual trailing edge cannot be strictly detected.
Further, the method discussed in Japanese Patent Application Laid-Open No. 09-301573 determines timing of feeding the following sheet based on the detection of the trailing edge of the sheet. Therefore, there has been no problem of sheet feed timing generation even if the trailing edge cannot be strictly detected.
However, to perform the automatic zooming function, it is necessary to accurately detect the sheet size and therefore detect the trailing edge with high accuracy. For this reason, the method discussed in Japanese Patent Application Laid-Open No. 09-301573 cannot be used as it is.